I believe merely having the Amex Hilton prevents expiration.

I have held a bronze (no fee) AA card for a decade that ran my ez-pass through (an occasional re-occurring bill) for the sole purpose of keeping miles alive over a decade when I stopped travelling for biz. But, as stated, you need transactions to post to keep them alive. It was a no hassle way to go, and its now my oldest active line of credit.